Effects of elevated temperatures during interruption of irradiation on Harwell Red 4034 PMMA and Kodak Biomax alanine film dosimetry systems

In the industrial setting it is not uncommon for a process interruption to occur during irradiation. In this event, dosimeters may be exposed to prolonged periods of elevated temperature without exposure to ionizing radiation. Once the process is restarted, the same dosimeters are exposed to ionizing radiation in order to achieve target dose. The goal of this experiment was to simulate a process interruption within limits and quantify the effects of a combination of factors (heat, time, and fractionation) on dosimeter response. We present an in-depth experimental study on the response of dosimeters that have been irradiated, stored for a fixed period of time at several temperatures, and then re-irradiated. This study was performed using Harwell Red 4034 polymethylmethacrylate (PMMA) and Kodak BioMax alanine film dosimeters.     

Characterization of a new photo-fluorescent film dosimeter for high-radiation dose applications

Characterization studies on one of the first versions of the Sunna fluorescent dosimeter™ have been published by Kovács and McLaughlin. This present study describes testing results of a newer version of the dosimeter (Model γ, batch 0399-20). This dosimeter is a 1-cm×3-cm polymeric film of 0.5 mm thickness that emits a green fluorescence component at intensities almost linear with dose. The manufacturing method (injection molding) allows potential batch sizes on the order of a million while maintaining a signal precision on the order of ±1%. Studies include dose response, dose rate dependence, energy dependence, post-irradiation stability, environmental effects, and variation of response within a batch. Data for both food irradiation and sterilization dose levels were obtained. The results indicate that the green signal (0.3–250 kGy) works well for food irradiation dose levels, especially in refrigerated facilities that maintain tight temperature control. The green signal also works well in sterilization facilities because its irradiation temperature coefficient above room temperature is minimal at sterilization doses. If the user requires readout results in < 22 h after room temperature irradiation, the user can either calibrate for a specific post-irradiation readout time(s) or simply heat the dosimeters in a small laboratory oven to quickly stabilize the signal.     

Uncertainties in absorbed dose as measured using PMMA dosimeters

Some Harwell quality-control data on red 4034 PMMA dosimeters are presented, showing that gaussian distribution and probability apply to the specific absorption measurements used to derive dose.

Uncertainties of calibration and use are listed, quantified, and combined in simple quadrature, for an example batch of dosimeters. The overall uncertainty of calibration derived from this listing is shown to agree with the average uncertainty found in a closed-loop inter-laboratory comparison exercise.     

Effects of humidity and light on dosimetric properties of clear polymethylmethacrylate

Effects of humidity and light conditions /fluoroscent light, sunlight and diffuse sunlight/ on the response of 2 mm thick clear polymethylmethacrylate /PMMA/ dosimeter during irradiation and during postirradiation storage were determined spectrophotometrically at 305 and 314 nm. The results showed that irradiation of dosimeters at 12–97% relative humidity did not result in any change in the response upto 2 weeks of post-irradiation storage. However, there is some decrease in the response at higher humidities />76%/ for longer storage time. Post-irradiation storage and dosimeters at constant relative humidity conditions between 35 and 97% had no effect on the response up-to 3 weeks. Post-irradiation storage at lower relative humidity /12%/ showed some decrease in the response for longer periods. The response of dosimeter is not very sensitive to short time exposure to diffuse sunlight and fluorescent light. However, a significant increase in the optical density was observed during exposure of dosimeters to direct sunlight.     

Selfcalibrated alanine/EPR dosimeters: A new generation of solid state/EPR dosimeters

Alanine/EPR dosimeters are well established as secondary, reference dosimeters for high-energy radiation. However, there are various sources of uncertainty in the evaluation of absorbed dose. This arises primarily from the necessity to calibrate each EPR spectrometer and each batch of dosimeters before their use. In order to overcome this disadvantage, a new generation alanine/EPR dosimeter has been developed, and its possibilities as a radiation detector are reported. Principally, it is a mixture of alanine, some quantity of EPR active substance, and a binding material. The EPR active substance, acting as an internal EPR standard, is chosen to have EPR parameters which are independent of the irradiation dose. The simultaneous recording of the spectra of both the sample and the standard under the same experimental conditions and the estimation of the ratioI _alanine/I _Mn as a function of the absorbed dose strongly reduces the uncertainties. The response of these dosimeters for⁶⁰Co γ-radiation exhibits excellent linearity and reproducibility in the range of absorbed dose, 10²−5·10⁴ Gy.     

Development of polystyrene calorimeter for application at electron energies down to 1.5 MeV

Polystyrene (PS) calorimeters developed at Riso National Laboratory for use below 4 MeV have been modified due to irradiation technology requirements concerning both design principles and dimensions. The temperature–time relationship after irradiation was measured, and two ways of dose measurement were tested: (1) real time temperature measurement during the irradiation and (2) pre- and post-irradiation temperature measurement. The advantages and drawbacks of these methods are discussed.Depth dose measurements have been carried out in the PS calorimeter to define the relationship between the average and the surface dose and to prove the applicability of the new low energy calorimeter for calibration purposes at 1.5 and 2 MeV electron energy. Alanine dosimeters of 2 mm thickness were used to calibrate the calorimeters and their use for nominal dose measurements was demonstrated in a series of intercomparisons. The use as routine dosimeters at electron accelerators operating in the energy range of 1.5–4 MeV was also demonstrated.     

Dosimetry characteristics of the nitro blue tetrazolium-polyvinylalcohol film for high dose applications

The dosimetry characteristics of a polyvinylalcohol based radiochromic dye film containing the ditetrazolium salt nitro blue tetrazolium chloride were studied with respect to the potential use of the films for routine dosimetry in radiation processing. The useful dose range for the dosimeter film for gamma and electron irradiation is 5–50 kGy depending on the concentration of the dye. The effects of irradiation temperature and humidity, as well as the stability of the response of the film before and after irradiation, were investigated and determined. Formulations for preparation of the films with different concentrations of the dye and with different pH were tested. The films were also tested in industrial gamma irradiation facilities for process control purposes by comparing their performance with transfer standard dosimeters.     

Effect of post-irradiation thermal treatments on the stability of gamma-irradiated glass dosimeter

A commercial window glass has been investigated as a routine high dose dosimeter for gamma irradiation. The irradiated samples showed rapid fading at room temperature immediately after irradiation. This short-term rapid fading was followed by a slow fading at long-term. This strong initial fading is a problem for dosimetry purposes. However, when the dosimeter is measured at the same time interval after irradiation, it maintains proportionality to dose. Calibration curves have to be used for different time intervals after irradiation. In order to improve post-irradiation stability dosimeters were submitted to different post-irradiation thermal treatments from (−20) up to 150 °C. After that, optical absorbance measurements were carried out up to 2 months at room temperature. The heating at 150 °C for 20 min was found to be the most suitable procedure for the removal of unstable entities responsible for the initial rapid fading. Due to these heat treatments, variation of response was found almost negligible 24 h after irradiation for several months. Calibration curves demonstrated the applicability of this glass as routine dosimeter in the dose range of 0.5–90 kGy.     

Dose response and post-irradiation characteristics of the Sunna 535-nm photo-fluorescent film dosimeter

Results of characterization studies on one of the first versions of the Sunna photo-fluorescent dosimeter™ have previously been reported, and the performance of the red fluorescence component described. This present paper describes dose response and post-irradiation characteristics of the green fluorescence component from the same dosimeter film (Sunna Model γ), which is manufactured using the injection molding technique. This production method may supply batch sizes on the order of 1 million dosimeter film elements while maintaining a signal precision (1σ) on the order of ±1% without the need to correct for variability of film thickness. The dosimeter is a 1 cm×3 cm polymeric film of 0.5-mm thickness that emits green fluorescence at intensities increasing almost linearly with dose. The data presented include dose response, post-irradiation growth, heat treatment, dosimeter aging, dose rate dependence, energy dependence, dose fractionation, variation of response within a batch, and the stability of the fluorimeter response. The results indicate that, as a routine dosimeter, the green signal provides a broad range of response at food irradiation (0.3–5 kGy), medical sterilization (5–40 kGy), and polymer cross-linking (40–250 kGy) dose levels.     

Three-dimensional polymer gel dosimetry: basic physical properties of the dosimeter

This study concentrated on assessment of the basic physical properties of a polymer gel dosimeter evaluated by NMR. For this, BANG-2 type polymer gel was prepared. The dosimeters were irradiated by ⁶⁰Co gamma photons and by 4, 6 and 18 MV X-ray photons for doses in the range 0–50 Gy. The multi-echo CPMG sequence was used for the evaluation of T₂-relaxation times in the irradiated gel dosimeters. Dependence of 1/T₂ in terms of the following factors was studied: absorbed dose, energy of applied radiation, temperature during NMR evaluation, time between irradiation and NMR evaluation and strength of the magnetic field. An exponential dependence of the 1/T₂ response on absorbed dose in the range 0–50 Gy was observed, while in the range 0–10 Gy the data could be fitted by a linear function. This paper also describes the dependence of 1/T₂ response on: radiation energy, strength of magnetic field and time from irradiation of the dosimeters to NMR evaluation. Increase of gel dosimeter 1/T₂ response with the decrease of the temperature during NMR evaluation has been qantitatively described. The polymer gel dosimetry system used in this study proved that it is a reliable system for three-dimensional dose distribution measurement.     

The role of the spectral sensitivity curve in the selection of relevant biological dosimeters for solar UV monitoring

To estimate the risk of enhanced UV-B radiation due to stratospheric ozone depletion, phage T7 and uracil thin-layer biological dosimeters have been developed, which weight the UV irradiance according to induced DNA damage. To study the molecular basis of the biological effects observed after UV irradiation, the spectral sensitivity curves of the two dosimeters and induction of the two major DNA photoproducts, cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts ((6-4)PDs), in phage T7 have been determined for polychromatic UV sources. CPDs and (6-4)PDs are determined by lesion-specific monoclonal antibodies in an immunodotblot assay. Phage T7 and uracil biological dosimeters together with a Robertson-Berger (RB) meter have been used for monitoring environmental radiation from the polar region to the equator. The biologically effective dose (BED) established with the three different dosimeters increases according to the changes in the solar angle and ozone column, but the degree of the change differs significantly. The results can be explained based on the different spectral sensitivities of the dosimeters. A possible method for determining the trend of the increase in the biological risk due to ozone depletion is suggested.     

Applicability study on existing dosimetry systems to high-power Bremsstrahlung irradiation

Applicability of the existing dosimetry systems to high-power Bremsstrahlung irradiation was investigated through a dose intercomparison study, where several dosimeters were irradiated in the dose range 4–12 kGy in identical polyethylene phantoms in a Bremsstrahlung beam obtained from a 5-MeV electron accelerator. Included in the study were alanine dosimeters molded by three different binders, three types of liquid dosimeters—ceric-cerous, dichromate and ethanol-chlorobenzen (ECB), and glutamine powder. The dosimeter responses for Bremsstrahlung radiation were analyzed at the issuing laboratories, and the dose values determined using calibration based on cobalt-60 gamma-ray irradiation. Dose values for all the three dose levels for all dosimetry systems were in good agreement—better than 3%. The results of the study demonstrate that these existing dosimetry systems have a potential for application to high-power Bremsstrahlung irradiation.     

Non-disturbing thermoacoustic dosimetry of electron and bremsstrahlung beams

The thermoacoustic response of thin targets (TTs) to pulsed electron/gamma irradiation is analyzed in the context of determination of the radiation characteristics. Basic equations of thermoacoustic dosimetry linking thermoacoustic stress amplitude to deposited energy density are presented. Using these equations, it is possible to restore the particle distribution in the pulsed beam from the thermoacoustic response of one-dimensional (wire, rod) or two-dimensional (film, plate) TTs. Generation coefficients for the TT-based dosimeters are presented. The construction and principles of operation of thermoacoustic TT-based dosimeters for determination of the characteristics of pulsed electron beams are considered. The availability of thermoacoustic gamma dosimetry is analyzed. Specific features and merits of the method of thermoacoustic dosimetry are discussed.     

Enhancement in sensitivity of nitro blue tetrazolium polyvinyl alcohol film dosimeters by sodium formate and Triton X-100

Nitro blue tetrazolium polyvinyl alcohol film dosimeters, NBT-PVA were prepared and evaluated based on radiation-induced reduction of NBT²⁺. NBT-PVA film dosimeters containing different concentrations of NBT dye from 1 to 5 mM were prepared in a solution of ethanol. The dosimeters were irradiated with ?-ray from ⁶⁰Co source at doses up to 50 kGy. UV/vis spectrophotometer was used to investigate the optical density of un-irradiated and irradiated films in terms of absorbance at 529 nm. The absorbance increases with absorbed dose up to 50 kGy for NBT-PVA film dosimeters. The dose sensitivity of NBT-PVA film increases strongly with increase of concentrations of NBT dye. The effects of irradiation temperature, humidity, dose rate and the stability of the response of the films after irradiation were investigated. A considerable increase was observed in the dose response of NBT-PVA film by adding appropriate concentration of sodium formate and Triton X-100.     

Optimization of the sensitivity and stability of the PRESAGE™ dosimeter using trihalomethane radical initiators

The aim of this study is to investigate the effect of trihalomethane radical initiators on the radiological properties, radiation dose sensitivity and post response photo-stability of the PRESAGE dosimeter. Different PRESAGE dosimeters containing 50 and 100 mM of iodoform (CHI₃), bromoform (CHBr₃) or chloroform (CHCl₃) radical initiators where fabricated and irradiated with 6 MV photons for a range of radiation doses from 0 to 30 Gy. A comparison between sensitivity and radiological properties of the PRESAGE dosimeters with the different radical initiators was carried out. Optical density changes of the dosimeters before and after irradiation were measured using a spectrophotometer. The incorporation of different radical initiators in the composition of the PRESAGE dosimeter resulted in variation of the radiation dose sensitivity and radiological properties of the dosimeters depending on the type and concentration of the radical initiator used, with iodoform showing the highest dose-response slope followed by bromoform and chloroform. However, at 100 mM iodoform, the effective atomic number was significantly higher than water (Z_eff=16). This enhancement in dose-response was found to be directly related to the carbon–halogen bond dissociation energy and to the radiological properties of each individual radical initiator used in this study. Furthermore, the post-response stability of the PRESAGE dosimeters over two weeks remained stable regardless of the trihalomethane radical initiator employed, with negligible change in the post-response stability and linearity of the PRESAGE dosimeters.     

A new label dosimetry system

A new label dosimeter which changes its color by large radiation doses has been developed. The green color of the unirradiated dosimeter gradually turns to brown then to red at high doses. The label dosimeter was prepared with a peal-off paper backing, allowing it to stick by self-adhesion to a product box. Three types of dosimeters having different sensitivities to radiation doses were prepared. Correlations were established between absorbed doses and color scale or the green/red axis of the irradiated dosimeters, using a micro color unit equipped with a data station. The data were analyzed to determine the reproducibility of the reflectance measured from the label dosimeters exposed to different doses of γ radiation. These dosimeters showed great stability on extended storage before and after irradiation.Detailed measurements of absorbed dose extremes (D_min and D_max) in product boxes, processed in the Egypt Mega Gamma I radiation processing facility, were obtained using these dosimeters. These dosimeters are currently available in large quantities and are inexpensive, which makes them suitable for routine high-dose applications in radiation processing of materials.     

Photo-luminescence of Risø B3 and PVB films for application in radiation dosimetry

Risø B3 film dosimeters (23 μm) prepared from poly(vinyl butyral) (PVB) incorporating pararosaniline cyanide, as the radiation-sensitive element and PVB films (25 μm) prepared from PVB without any additives are investigated for γ-radiation measurement using spectrofluorimetry based on their emission properties. The unirradiated Risø B3 film when excited at 554 nm shows an emission band at 602 nm while PVB film shows an emission band at 305 nm when excited at 235 nm wavelength. The fluorescence intensity of both emission bands decreases with the increase of absorbed dose due to the damage caused by ionizing radiation. The useful dose range of Risø B3 film extends up to 120 kGy while that of PVB film extends up to 60 kGy. The response of Risø B3 film increases with the increase of relative humidity during irradiation while that of PVB has less effect in the humidity range of 20–70%. The percent uncertainty associated with the measurement of the dose response was found to be ±3% (1σ) for both films. Risø B3 and PVB films show good post-irradiation stability in dark and indirect daylight where the deviation in the response overall a 2-month storage period was found to be ±5% for Risø B3 and ±2% for PVB.     

Temperature response of a number of plastic dosimeters for radiation processing

Various plastic dosemeters are employed for dosimetry control of radiation processing within gamma and electron irradiation facilities.

The temperature response of a dosimeter is important when the dose to such a dosimeter is accumulated under varying irradiation temperatures. Such measurements would be significant for proper assessment of the dose for better process control, as well as, performance evaluation of dosimetry systems.

In this work we have developed a high current peltier junction temperature controller system for our Gammacell-220. This system has been designed to regulate the operating temperature of the irradiation chamber in the range of 0 to 80 C this system has been applied to measure the temperature response of the red perspex, a local clear PMMA, Gammex, Gammachrome, and Gafchromic dosimeters. The curves of relative performance or variation of the induced optical densities of the above dosemeters versus the irradiation temperature at fixed dose values are obtained.     

LET effects following neutron irradiation of lithium formate EPR dosimeters

Lithium formate electron paramagnetic resonance (EPR) dosimeters were irradiated using 60Co gamma-rays or fast neutrons to doses ranging from 5 to 20 Gy and investigated by EPR spectroscopy. Using a polynomial fitting procedure in order to accurately analyze peak-to-peak line widths of first derivative EPR spectra, dosimeters irradiated with neutrons had on average 4.4+/-0.9% broader EPR resonance lines than gamma-irradiated dosimeters. The increase in line width was slightly asymmetrical. Computer simulated first derivative polycrystalline EPR spectra of a *CO2- radical gave very good reconstructions of experimental spectra of irradiated dosimeters. The spectrum simulations could then be used as a tool to investigate the line broadening observed following neutron irradiation. It was shown that an increase in the simulated Lorentzian line width could explain both the observed line broadening and the asymmetrical effect. The ratio of the peak-to-peak amplitude of first derivative EPR spectra obtained at two different microwave powers (20 and 0.5 mW) was 7.8+/-1.2% higher for dosimeters irradiated with neutrons. The dependence of the spectrum amplitude on the microwave power was extensively investigated by fitting observations to an analytical non-linear model incorporating, among others, the spin-lattice (T1) and spin-spin (T2) relaxation times as fitting parameters. Neutron irradiation resulted in a reduction in T(2) in comparison with gamma-irradiation, while a smaller difference in T1 was found. The effects observed indicate increased local radical density following irradiation using high linear energy transfer (LET) neutrons as compared to low LET gamma-irradiation. A fingerprint of the LET may thus be found either by an analysis of the line width or of the dependence of the spectrum amplitude on the microwave power. Lithium formate is therefore a promising material for EPR dosimetry of high LET radiation.     

E-beam curing of epoxy-based blends in order to produce high-performance composites

In this work, blends of a difunctional epoxy monomer and a thermoplastic toughening agent are E-beam irradiated at two different dose rates and two different total absorbed doses. The influence of the processing conditions on the thermal properties and on the morphology of the obtained matrices has been investigated. In particular, it is shown how the increase of the dose rate causes an increase of the temperature during irradiation, thus inducing a simultaneous thermal and radiation curing. On the contrary, at low-dose rate the system mainly undergoes to radiation curing, thus making the cured material very sensible to a post-irradiation thermal treatment with a significant improvement of the thermal properties.